Add Neo-Hookean material with properties defined at points and sinker test

Summary

Implements a basic neo-Hookean material model with \mu, k, and \rho defined at points. The material properties are mapped from the initial cells the points are contained in, then remain constant on the particles for the remainder of the simulation.

Sinker test:

Shown with order 2, 27 MPM points per cell.

Points, colored by z displacement (cut through center) with diagnostic mesh:

Points, colored by z displacement (cut through center):

Checklist

Closes #145 (closed) and addresses part of #194

• Atomic commit history
• All new features documented
• Updated CHANGELOG.md